Embodiments of the present invention relate to a non-volatile memory device, and more specifically, to a phase change memory device for performing a sensing operation using a current signal.
Memory devices can be classified into a volatile memory device and a non-volatile memory device. The non-volatile memory device includes a non-volatile memory cell capable of preserving stored data even when not powered. For example, the non-volatile memory device may be implemented as a flash random access memory (flash RAM), a phase change random access memory (PCRAM), or the like.
The PCRAM includes a memory cell that is implemented using a phase change material such as germanium antimony tellurium (GST), wherein the GST changes to a crystalline phase or an amorphous phase if heat is applied to the GST, thereby storing data in the memory cell.
A non-volatile memory device (e.g., a magnetic memory, a phase change memory (PCM), or the like) has a data processing speed similar to that of a volatile RAM device. The non-volatile memory device also preserves data even when power is turned off.
FIGS. 1A and 1B illustrate a conventional phase change resistor (PCR) element 4.
Referring to FIGS. 1A and 1B, the PCR element 4 includes a top electrode 1, a bottom electrode 3, and a phase change material (PCM) layer 2 located between the top electrode 1 and the bottom electrode 3. If a voltage and a current are applied to the top electrode 1 and the bottom electrode 3, a current signal is provide to the PCM layer 2, and a high temperature is induced in the PCM layer 2, such that an electrical conductive status of the PCM layer 2 changes depending on resistance variation.
FIGS. 2A and 2B illustrate a phase change principle of the conventional PCR element 4.
Referring to FIG. 2A, if a low current smaller than a threshold value flows in the PCR element 4, the PCM layer 2 has a temperature suitable for a crystalline phase. Therefore, the PCM layer 2 changes to the crystalline phase, which is a low-resistance phase material. As a result, a current may flow between the top electrode 1 and the bottom electrode 3.
On the other hand, as shown in FIG. 2B, if a high current greater than the threshold value flows in the PCR element 4, the PCM layer 2 has a temperature higher than a melting point. Therefore, the PCM layer 2 changes to an amorphous phase, which is a high-resistance phase material. As a result, it is difficult for the current to flow between the top electrode 1 and the bottom electrode 3.
As described above, the PCR element 4 can store data corresponding to two resistance phases as non-volatile data. For example, if the PCR element 4 has a low-resistance phase set to data ‘1’ and the PCR element 4 has a high-resistance phase set to data ‘0’, the PCR element 4 may store two logic states for data.
In addition, a phase of the PCM layer (i.e., a phase change resistive material) 2 is not changed although the phase change memory device is powered off, such that the aforementioned data can be stored as non-volatile data.
FIG. 3 illustrates a write operation of a conventional PCR cell.
Referring to FIG. 3, when a current flows between the top electrode 1 and the bottom electrode 3 of the PCR element 4 for a predetermined time, heat is generated.
Assuming that a low current smaller than a threshold value flows in the PCR element 4 during the predetermined time, the PCM layer 2 has the crystalline phase formed by a low-temperature heating state, such that the PCR element 4 becomes a low-resistance element having a set state.
Otherwise, assuming that a high current greater than the threshold value flows in the PCR element 4 during the predetermined time, the PCM layer 2 has the amorphous phase formed by a high-temperature heating state, such that the PCR element 4 becomes a high-resistance element having a reset state.
By means of the aforementioned properties, in order to write data of the set state during the write operation, a low voltage is applied to the PCR element 4 for a long period of time. On the other hand, in order to write data of the reset state during the write operation, a high voltage is input to the PCR element 4 for a short period of time.
The PCR memory device outputs a sensing current to the PCR element 4 during a sensing operation, such that it can sense data written in the PCR element 4.